The overarching goal of this competing renewal is to investigate systematically the mechanisms of stress- potentiated opioid seeking and biobehavioral responses. The proposed aims build programmatically on the very productive human laboratory paradigm and significant findings from this project's initial funding cycle. We combine a sensitive choice progressive ratio (exponentially escalating response requirement) procedure, well- established subjective drug effect and mood state measures, innovative hippocampal/prefrontal cortical vs. dorsal striatal-dependent learning tasks, and physiological indices of sympathetic- and HPA-mediated responses (i.e. heart period variability, blood pressure, body temperature, and salivary cortisol and 1-amylase). The primary drug-seeking outcomes will be analyzed using sensitive behavioral economic methods. Three proposed studies will systematically extend this useful laboratory model in significant, innovative and impactful directions (including validation of alternative stressors). The proposed study designs are sound and employ within-subject, randomized crossover, placebo-controlled, double-blind methodology. Our goal is to advance theoretical understanding (e.g. biobehavioral responses should be differentially sensitive to the stressors) and generate hypotheses for practical applications (e.g. medication development). Aim 1. Fully characterize yohimbine-potentiated opioid seeking and biobehavioral responses using an optimized model (methods adjusted based on Prelim. Study 4 findings) and compare to a naturalistic stressor (moderately loud, intermittent, inescapable soundtrack of crying/distressed infants). Aim 2. Use an alternative neuropharmacological stressor paradigm (reboxetine/hydrocortisone dose combinations) to determine whether activation of noradrenergic (Not applicable) and glucocorticoid transmission increases opioid seeking and biobehavioral responses. Aim 3. Determine whether neuromodulating agents at 12A-adrenergic (guanfacine 1 mg), 5-HT1A (buspirone 30 mg), CB1 (cannabidiol 1000 mg), and GABAB (baclofen 40 mg) receptors differentially modulate yohimbine- potentiated opioid seeking and biobehavioral responses.